What is the guarantee of uniqueness of shortid?

I'm trying to include a field in Mongodb document called myId. I am using shortid. I am wondering, in case of big data, like millions of documents in a collections:

1. What's the guarantee that the shortid will be always unique and never ever be repeated for any other document?
2. What keeps a track of the generated ids?
3. What are the chances of the id been repeated?

via Somename